Polystyrene foam made with only carbon dioxide as a blowing agent and a process for making the same

ABSTRACT

The present invention is a styrenic polymer foam blown with solely carbon dioxide and a process for making that styrenic polymer foam. Foam sheet made by this process is especially useful for thermoforming purposes.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of application Ser. No. 07/388,468 filed Aug. 2,1989 U.S. Pat. 5,250,577.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to polystyrene foam prepared with a blowing agentwhich is only carbon dioxide. The invention also includes a process formaking this polystyrene foam utilizing only carbon dioxide as theblowing agent. This invention is particularly suited for makingpolystyrene foam sheet having a thickness of less than about 0.5 inch.

2. History of the Art

Polystyrene foam, particularly in sheet form, is at the present timemade from any number of blowing agents which may possess or cause one ormore of the following undesirable characteristics: volatility,flammability, poor thermoforming characteristics, brittle foam sheetphysical properties, high cost or an adverse effect to the ozone layer.Examples of these blowing agents would include aliphatic hydrocarbonsand fully or partially halogenated hydrocarbons.

Some have experimented with blends of carbon dioxide and other blowingagents such as aliphatic hydrocarbons or fully or partially halogenatedhydrocarbons. An example of this can be seen in U.S. Pat. Nos. 4,344,710and 4,424,287. These patents, however, state that the use of a 100percent carbon dioxide blowing agent has not been sucessfully employedin continuous direct injection foam sheet manufacturing (Column 1, lines42-45 and lines 49-55 respectively) due to the extreme volatility. Useof these materials is said to produce corrugation and surface defects.

Others have developed methods and apparatuses for metering a gaseousblowing agent, such as an atmospheric gas, into a molten resin charge,such as polystyrene, in an accurate and precise manner. One example ofthis can be seen in U.S. Pat. No. 4,470,938. The apparatus of thispatent meters discrete uniform volumetric charges of gaseous blowingagent into the molten resin using a free piston-cylinder combinationwhich is given motive power by the supply gas source.

A typical current commercial polystyrene foam sheet extrusion processmay use a tandem extrusion process (two extruders in series). The firstextruder melts the polymer to produce a polymer melt. A high pressuremetering pump would then deliver blowing agent to the polymer melt atthe end of the first extruder where mixing is initiated prior toentering the second extruder where further mixing and cooling of theblowing agent and the polymer melt occurs. After exiting the secondextruder the polymer melt then passes through and becomes a foamstructure at an annular die. The foam structure, in the shape of anannular tube, is then stretched over a forming mandrel. The annular tubecoming off the mandrel is then slit and opened to form a sheet which isthen gathered on one or more rolls. The sheet is then aged and thenthermoformed into a foam sheet packaging article.

The use of carbon dioxide as the sole blowing agent avoids many of theproblems of other blowing agents and blends of other blowing agent withcarbon dioxide. The use of carbon dioxide in liquid form avoids theproblems of handling a gas.

Thus the invention is polystyrene foam blown solely with carbon dioxide,preferably liquid carbon dioxide, and the process necessary to make suchpolystyrene foam.

The invention is particularly suited for making polystyrene foam sheethaving beneficial physical properties for thermoforming purposes.

SUMMARY OF THE INVENTION

The present invention is a thermoplastic foam made from a styrenicpolymer and carbon dioxide as a blowing agent with the thermoplasticfoam having tensile elongation values greater than or equal to aboutfive percent over an extended period of time under ambient conditions inboth a machine and a cross direction.

The present invention is also a foamable mixture of a molten styrenicpolymer and carbon dioxide as a blowing agent.

Additionally the present invention is a process for producing athermoplastic foam (and the foamable mixture) by melting a styrenicpolymer, then continuously directly introducing into the melted styrenicpolymer a blowing agent consisting essentially of carbon dioxide, thenmixing intimately the styrenic polymer and the carbon dioxide and thenextruding and foaming the mixture at a die temperature below about 150°C. into a region of lower pressure to form thermoplastic foam.

Finally the present invention also includes thermoplastic foam made bythe process of the present invention.

The thermoplastic foam and process of the present invention areespecially suited to make thermoplastic foam sheet for thermoformingpurposes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are machine direction and cross direction tensileelongation values, respectively, for foams prepared with carbondioxide(CO₂) and dichlorodifluoromethane (CFC-12) as blowing agents.

FIG. 3 is a post expansion study of foam sheet at three differentresidence times in an oven.

DETAILED DESCRIPTION OF THE INVENTION

Polymers useful in the present invention are those polymers, copolymersand interpolymers having at least 70 percent by weight styrene monomerincorporated into the final resin as the monomeric repeat units. Thisincludes other polymers such as for example styrene acrylonitrile,styrene acrylic acid, other known equivalents and blends having thestyrene monomer repeat unit requirement. For the purpose of thisinvention these useful polymers shall be referred to as styrenicpolymers. The preferred materials are styrenic homopolymers.

The blowing agent to be used would be that carbon dioxide which isnormally commercially available. For the purposes of this specificationsuch carbon dioxide shall be defined as 100 percent carbon dioxide eventhough this carbon dioxide may contain minor impurities or amounts ofother compounds.

The carbon dioxide is preferably added to a polymer melt in a liquidform, although use of the carbon dioxide in the gaseous form would alsobe acceptable in the practice of the present invention.

The use of a tandem extrusion process for the manufacture of polystyrenefoam sheet is typical, but is not required. Such a process includes aprimary extruder, a blowing agent addition system into the primaryextruder, a secondary extruder, an annular die, a cooling mandrel, asheet cutter or slitter and a sheet gathering device, such as forexample rollers and winders.

However, the use of this exact equipment set-up is not required in theprocess of this invention.

Additives, such as pigments, nucleating agents and other additivesconventionally known in the art may be added to the polystyrene foam ofthe present invention. The addition of a nucleating agent is generallypreferred and is added in an amount of from about 0.02 to about 10percent of the total polystyrene by weight. Preferably the amount ofnucleating agent is between about 0.2 to about 2 percent. Oneconventional nucleating agent is talc.

Polystyrene foam sheet can be produced using 100 percent carbon dioxideas the blowing agent provided certain modifications are made in thehandling and the preparation of the polystyrene foam sheet. Preferablythe carbon dioxide is continuously directly introduced or injected intothe molten resin. Preferably the carbon dioxide is continuously directlyinjected as a high pressure liquid.

To produce foam sheet having a density in the range of about 2 to about10 pounds per cubic foot (PCF), preferably about 3 to about 6 PCF, about0.5 to about 6 weight percent, preferably about 1 to about 4 weightpercent of carbon dioxide is required per weight of polymer. Cell sizesin all directions are less than 1 millimeter(mm), and preferably lessthan about 0.5 Mm. The thickness of the foam sheet is preferably lessthan about 0.5 inch and more preferably less than about 0.25 inch.

It is possible to have a blow up ratio, that is a stretching of the foamover a mandrel after it exits a die, greater than 4:1. It is preferableto have a blow up ratio of at least 2.0:1.

The foaming temperature at the die is below 150° C. and preferablybetween 130° and 145° C. The thermoplastic foam is extruded and foamedinto a zone of lower pressure preferably air at atmospheric pressure.

While it may be possible to use a polystyrene foam sheet immediately itis preferred to age the polystyrene foam sheet for a period of time,usually at least about 20 hours prior to thermoforming the foam sheet.This will allow time for air to counter diffuse into the cells. It willalso render the tensile elongation values substantially constant. Thiswill provide uniform thermoforming characteristics for the polystyrenefoam sheet to be thermoformed.

Substantially constant tensile elongation values over an extended periodof time obtained with a minimum of aging of the foam sheet would yieldmore consistent results upon thermoforming the foam sheet, thus moreconsistent product with less scrap. The tensile elongation values wouldpreferably be in excess of five percent and would remain above fivepercent for an extended period of time, thus allowing the storage offoam sheet for an extended period of time before thermoforming withoutworrying about the changing values or the degradation below five percentof the tensile elongation values of the foam sheet.

The tensile elongation values are determined according to the methodASTM D638 "Tensile Properties of Plastics" published by the AmericanSociety for Testing and Materials (ASTM). The words machine or extrusiondirection, transverse or cross direction and vertical direction are usedin their normal and conventional manners.

Though the tensile elongation values of the foam sheet of the presentinvention are not perfectly constant, they are substantially constantand even more important for thermoforming, the tensile elongation valuesremain above a value of five percent for an extended period of time.Generally when these values fall below five percent the foam sheetbecomes too brittle for thermoforming.

The thermoforming temperature will be somewhat higher than that of apolystyrene foam sheet blown with fully or partially halogenatedchlorofluorocarbon or hydrocarbons since the CO₂ provides essentially nosolvent effect which would normally lower the heat distortiontemperature and therefore the melt strength of the foam walls uponreheating. The polystyrene foam sheet blown with 100 percent CO₂ can bepost expanded in a range of about 20 to about 50 percent duringthermoforming. It is preferred that the polystyrene foam sheet be agedfor at least 20 hours prior to the carefully controlled thermoforming ofthe foam sheet.

EXAMPLE AND COMPARATIVE EXAMPLE

Polystyrene foam sheet is prepared on a tandem extrusion line, with thepolystyrene resin having a weight average molecular weight of about300,000 being melted in the first extruder and with the carbon dioxidebeing continuously directly injected as a high pressure liquid into theend of the first extruder as a liquid. The pump heads providing thecarbon dioxide to the extruder are cooled to about 0° F. with liquidnitrogen to eliminate vapor lock and to improve pump efficiency. Thecarbon dioxide is metered using a mass flow meter on a low pressure sideor suction side of the pump. Confirmation of the percent of CO₂ used isalso made by measuring the weight loss of the CO₂ from storage cylindersholding the CO₂.

The results of this example and a comparative example, conventionallyblown with dichlorodifluoromethane (CFC-12), are shown in Table I.Weights are percent weight by polymer weight. The thermoforming wasperformed in a manner typical for forming meat trays.

                  TABLE 1                                                         ______________________________________                                        Foam Sheet and Extrusion Process Parameters                                   100% CO.sub.2 Versus CFC-12                                                                    Comparative                                                                   Example  Example                                                              CFC-12   100% CO.sub.2                                       ______________________________________                                        % Blowing Agent by weight                                                                        5.0        2.5                                             % Talc by weight   0.7        0.5                                             Sheet Gauge, mils  120        130                                             Sheet Density, PCF 3.5        3.2                                             Die Melt Temperature, °C.                                                                 150        144                                             Output Rate, lbs/hr                                                                              198        169                                             Blow Up Ratio      3.5:1      3.5:1                                           Extruder Pressure, psi                                                                           3800       4700                                            Die Pressure psi   2000       3500                                            Sheet Aging Time, days                                                                           7          3                                               Thermoformer % Post Expansion                                                                    54         30                                              Tooling Used       meat tray  meat tray                                       ______________________________________                                    

As can be seen from Table 1 it is possible to use a lower amount ofcarbon dioxide to produce a foam sheet having approximately the samethickness and same density as a foam polystyrene foam sheet blown withthe fully halogenated chlorofluorocarbon CFC-12. In addition the foamsheet produced from the 100 percent CO₂ blowing agent under theseconditions has a smooth, uniform surface similar to that of foam sheetmanufactured with aliphatic hydrocarbons and fully or partiallyhalogenated hydrocarbons.

A typical set of cell sizes for 100 percent CO₂ blown foam sheet in themachine direction is 0.39 mm; cross direction is 0.33 mm; verticaldirection is 0.28 mm. For the CFC-12 blown foam sheet, typical cellsizes are in the machine direction 0.28 mm; in the cross direction 0.33mm; in the vertical direction 0.22 mm.

FIGS. 1 and 2, with a different example and comparative example (densityof 5.1 PCF for the CO₂ blown foam sheet and 5.4 PCF for the CFC-12 blownsheet), also show an unexpected benefit when foam sheet is producedusing only carbon dioxide as the blowing agent. The benefit is that thepercent elongation in both the machine (FIG. 1) and transverse (FIG. 2)directions remains essentially constant, or at worst, decreases onlyslightly over time after a short initial aging period.

As FIGS. 1 and 2 show the tensile elongation values remain substantiallyconstant for the CO₂ blown foam sheet, with only a 9.3 percent change inthe machine direction from the period of time of 24 hours to 313 hoursand only a 7.4 percent change in the cross direction during the sameperiod.

In contrast, when using CFC-12 as the blowing agent, the percentelongation decreased drastically and substantially over the same timeperiod after the same short initial aging period. FIGS. 1 and 2 showthat the CFC-12 blown foam changed 29.0 percent in the machine directionand 15.2 percent in the cross direction for the period of time of 18.5hours to 330 hours.

Also in FIGS. 1 and 2 the percent elongation in both directions is abovefive percent for an extended period of time only when carbon dioxide isused as the blowing agent.

The CO₂ blown foam sheet exhibits a machine direction tensile elongationof 6.8 percent and a cross direction tensile elongation of 6.3 percentafter 313 hours of aging. The CFC-12 blown foam has a machine directiontensile elongation of 4.4 percent and a cross direction tensileelongation of 3.0 percent after 330 hours of aging.

Tensile elongation values below five percent are generally considered tobe brittle and not acceptable for thermoforming purposes.

Tensile elongation values for the foam sheet which change at a rapidrate over an extended time make a determination as to when to thermoformthe foam sheet difficult, and in addition, the foam sheet becomesbrittle and difficult or impossible to thermoform when the tensileelongation values are less than five percent.

The product of the present invention has elongation values which remainconstant, or at worse, decline only slightly over a long period of time.This means, as shown in FIGS. 1 and 2, that foam sheet stored on a rolland waiting to be thermoformed will have approximately the sameelongation values at the thirty hour mark as at the one hundred sixtyhour mark. This is not true for foam sheet produced withdichlorodifluoromethane and other types of aliphatic hydrocarbon andfully or partially halogenated hydrocarbon blowing agents.

CORRUGATION

It is also possible with the present invention to make 3-6 PCF foamsheet blown with CO₂ having little or no corrugation.

For the purpose of definition "little or no corrugation ornoncorrugated" means that the thickness values for peaks and valleys areaveraged, the overall average is determined by averaging the peak andvalley averages and the difference between the overall average and thepeak and valley average is less than about fifteen (15) percent.

In Table 2 Sheets 1, 2 and 3 have a blow-up ratio of 3.5:1. The weightpercent CO₂ is 2.3 for Sheet 1; 2.0 for Sheet 2; and 1.3 for Sheet 3.The die temperature is 146° C. (centigrade) for Sheet 1; 141° C. forSheet 2; and 144° C. for Sheet 3.

                  TABLE 2                                                         ______________________________________                                        Sheet 1          Sheet 2         Sheet 3                                      3.77 PCF         3.60 PCF        5.75 PCF                                     Peak  Valley     Peak    Valley  Peak  Valley                                 ______________________________________                                        141   94         105     84      87    81                                     154   101        105     88      88    85                                     171   108        108     89      92    99                                     127   89         103     90      87    95                                                                      85    86                                     AVG.  AVG.       AVG.    AVG.    AVG.  AVG.                                   148   98         105     88      88    89                                     Average of                                                                    Peak & Valley                                                                 123              96.5            88.5                                         Difference                                                                    Based on AVG                                                                  20 percent       8.8 percent     0.6 percent                                  ______________________________________                                    

As can be Table 2, with the values in mils (0.001 inch), the corrugationof Sheet 1 is 20 percent. This corrugation is visibly noticeable andthis level of corrugation makes this sheet unsuitable for manythermoforming operations. Sheets 2 and 3 are acceptable. As can be seenthe corrugation is 8.8 and 0.6 percent respectively, well below the 15percent level. The difference between Sheet 1 and Sheets 2 and 3 isvisibly noticeable and measurements confirmed the visual evidence.

As is clear in these examples, corrugation can be minimized or evenprevented by controlling the die temperature and amount of blowing agentwhen using 100 percent carbon dioxide as a blowing agent.

POST EXPANSION (THERMOFORMING)

Polystyrene foam sheet 5.0 PCF and 100 mils thick, blown with carbondioxide, is thermoformed into egg cartons in a thermoformer. Prior tobeing thermoformed the foam sheet is heated in an oven for 5, 6 or 7seconds as detailed in FIG. 3. The oven temperatures are about 500° F.(Fahrenheit) in the bottom of the oven and about 600° F. in the top ofthe oven.

FIG. 3 shows that although the percent of puff or post expansion issensitive to the amount of time in the oven, generally the amount ofpuff can be as much as about 20 to about 50 percent.

The present invention has been described with preferred embodiments. Itis to be understood however that modifications and variations may beresorted to, without departing from the spirit and scope of theinvention, as those skilled in the art will readily understand. Thesemodifications and variations are considered to be within the scope andpurview of the appended claims.

What is claimed is:
 1. A thermoplastic foam comprising:(a) a styrenicpolymer; and (b) a blowing agent consisting essentially of carbondioxidewherein the thermoplastic foam has tensile elongation valuesgreater than or equal to about five percent over an extended period oftime under ambient conditions in both a machine and a cross directionhas corrugation less than about fifteen percent, has a thickness lessthan about 0.5 inch and has a cell size, in any direction, of less thanabout one millimeter.
 2. A thermoplastic foam, as recited in claim 1,wherein the thermoplastic foam has substantially constant tensileelongation values over an extended period of time under ambientconditions in both a machine and a cross direction after an initialaging period.
 3. A thermoplastic foam, as recited in claim 2, whereinthe substantially constant tensile elongation values in either machineand a cross direction under ambient conditions vary by no more thanabout fifteen percent after an initial aging period of 20 hours.
 4. Athermoplastic foam, as recited in claim 1, wherein the thermoplasticfoam is in sheet form.
 5. A thermoplastic foam, as recited in claim 1,wherein the thermoplastic foam has a density of about 2 to about 10pounds per cubic foot.
 6. A thermoplastic foam, as recited in claim 5,wherein the thermoplastic foam has a density of about 3 to about 6pounds per cubic foot.
 7. A thermoplastic foam, as recited in claim 1,wherein the thermoplastic foam has a thickness less than about 0.25inch.
 8. A thermoplastic foam, as recited in claim 1, wherein thethermoplastic foam has a cell size, in any direction of less than about0.5 millimeter.
 9. A thermoplastic foam, as recited in claim 1, whereinthe thermoplastic foam has a blow-up ratio of at least 2.0:1.
 10. Athermoplastic foam, as recited in claim 1, wherein the thermoplasticfoam expands about 20 to about 50 percent upon heating prior tothermoforming.
 11. A thermoplastic foam, as recited in claim 1, whereinthe thermoplastic foam has carbon dioxide present in an amount of about0.5 to about 6 weight percent per weight of styrenic polymer immediatelyafter being extruded from a die and foamed.